1. Field of the Invention
The present invention relates to a radio frequency (RF) transmitter for a Wireless Local Area Network (Wireless LAN) device, and more particularly, to an RF transmitter for enhancing the output linearity of the Wireless LAN device.
2. Description of the Prior Art
Wireless communications products develop with the trend of low cost, small size, and high performance in modern life. Also, wireless communications protocols are getting complicated, so that it is difficult to properly design wireless communication products to meet many requirements. For example, power consumption of an 802.11n 2×2 Wireless LAN card is much higher than that of a former 802.11g Wireless LAN card. It will be a great challenge to design an 802.11n Wireless LAN card with high data transmission rate.
On the other hand, performance of a power supply has a direct influence on electrical equipment using it. When a switching regulator operates at a high frequency, it consumes less power and has 80%-90% efficiency, and thereby is suitable for Wireless LAN products for providing regulated DC power.
However, in the instant a large current is supplied to the Wireless LAN device, the output voltage of the switching power supply drops, and the operation voltage of the power amplifier drops accordingly, so that the output linearity of the Wireless LAN device is decreased. In addition, the power amplifier amplifies noise generated during the transition between transmission and reception, which also decreases the output linearity of the Wireless LAN device.
Please refer to FIG. 1, which is a schematic diagram of a Wireless LAN device 10 according to the prior art. The Wireless LAN device 10 can be a wireless communications product, such as a Wireless LAN card, or access point equipment. The Wireless LAN device 10 includes a voltage generator 101, a baseband processing unit 100, a radio frequency (RF) processing unit 102, an antenna 104, an RF receiver 106 and an RF transmitter 108. The voltage generator 101 is a switching regulator for generating a DC power. The baseband processing unit 100 is coupled to the voltage generator 101 and is utilized for processing a baseband signal and generating a control voltage Vctrl. The RF processing unit 102 is coupled to the voltage generator 101 and the baseband processing unit 100, and is utilized for performing transformation of the baseband signal into an RF signal.
The RF receiver 106 is coupled between the RF processing unit 102 and the antenna 104, and is utilized for receiving the RF signal via the antenna 104 and outputting the RF signal to the RF processing unit 102. The RF transmitter 108 is coupled to the baseband processing unit 100, the RF processing unit 102 and the antenna 104, and is utilized for transmitting the RF signal generated by the RF processing unit 102 to the air via the antenna 104. The RF transmitter 108 includes a power amplifier 180 and a resistor 182. The power amplifier 180 is coupled between the RF processing unit 102 and the antenna 104 and is driven by a bias Vref, for amplifying power of the RF signal. The resistor 182 is coupled between the baseband processing unit 100 and the power amplifier 180, and is utilized for transforming the control voltage Vctrl into the bias Vref.
Note that, in the conventional Wireless LAN device 10, the resistor 182 can only transform the control voltage Vctrl into the bias Vref, and cannot help reduce power noise in an input terminal of the power amplifier 180. Please refer to FIG. 2, which is an OFDM 64-QAM constellation of the Wireless LAN device 10 for an 802.11n Wireless LAN card. As shown in FIG. 2, deviations of signal points mean that error vector magnitude (EVM) of the output signals rises in the instant the voltage generator 101 supplies a large current, so that the output linearity and the data transmission rate of the Wireless LAN device 10 are decreased.
From the above, the conventional Wireless LAN device 10 cannot reduce power noise occurring in the input of the power amplifier 180. Therefore, noise will be amplified so that the output linearity of the Wireless LAN device 10 is decreased.